Delayed fracture occurs in iron and steel materials after a lapse of a certain time from the application of stress on the materials. Different causes may probably be tangled complicatedly to cause delayed fracture. This impedes the identification of the causes. In general, however, a common view is that a hydrogen embrittlement phenomenon is involved in delayed fracture. Independently, the tempering temperature, microstructure, material hardness, grain size, and alloy elements are recognized for the present as factors that influence the hydrogen embrittlement phenomenon. However, there is not established a process of preventing hydrogen embrittlement, and various processes or techniques have been proposed merely by trial-and-error methods under the present circumstances.
Exemplary techniques to improve the hydrogen embrittlement resistance of a bolt can be found in Patent Literature (PTL) 1 to 3 as a technique of adapting alloy elements. PTL 1 to 3 disclose that the adaptation of alloy elements gives a high-strength bolt having a high strength and still exhibiting excellent delayed fracture resistance. PTL 4 mentions that an alloy steel can have better hydrogen embrittlement resistance by quenching the alloy steel, tempering the alloy steel after quenching to allow fine alloy compounds to precipitate in large numbers, and allowing the precipitates to trap hydrogen (diffusible hydrogen) migrating in the steel.
A steel manufactured by the technique of adding large amounts of alloy elements as in PTL 1 to 4 exhibits excellent hydrogen embrittlement resistance (delayed fracture resistance) in an environment at a relatively low hydrogen level. The steel, however, becomes susceptible to delayed fracture contrarily in an environment at such a high hydrogen level as to consume all the hydrogen trapping sites and in an environment involving severe steel corrosion. This is because hydrogen once trapped by carbide particles acting as trapping sites is released from the trapping sites when the environment temperature changes and/or the steel stress varies. In the environments, therefore, hydrogen is released from the trapping sites under the circumstances to increase the diffusible hydrogen content in the steel.